You're right. That's always when I lose control--when I'm alone--and since no one else knows about my addiction, neither they nor I take any precautions before I'm by myself. What really worries me is that I keep pushing the limit. I know I'll eventually get caught, but I don't consider that when I'm looking at porn. I know it's wrong, even when I'm in the middle of it, but I always think, "Well, I'll stop after this one last time...".
I guess that's the definition of an addiction.